The present invention relates to a method for operating a linear compressor, in particular for a refrigerator. A linear compressor of this kind is known for example from U.S. Pat. No. 6,506,032B2 and U.S. Pat. No. 6,642,377B2. It comprises a reversing linear drive with a winding and an armature that can be displaced by a magnetic field generated by the winding against a spring force and a compression chamber, in which a piston is coupled to the armature in a displaceable manner. In operation, an alternating current is applied to the winding in order to drive an oscillating movement of the armature.
While with a conventional rotary-driven compressor the amplitude of motion of the piston is strictly specified, this is not the case with a linear compressor. The armature can oscillate with different amplitudes depending upon the electrical drive power supplied to the winding and accordingly the piston stroke is also variable.
The lower the drive power, and accordingly also the amplitude of the armature, the greater the dead volume of the pump chamber at the upper inversion point of the piston path. A large dead volume results in a low compressor efficiency since the work used to compress the gas in the dead volume is not used and, after overcoming the top dead center, the gas expands again and thereby drives the piston back.
If, on the other hand, the drive power applied to the winding is too high, the amplitude of the armature can become so high that the piston strikes a boundary of the compression chamber. This results in the development of a loud noise and possibly also damage to the compressor. In addition, the oscillation of the armature and the driving alternating current fall out of phase so that the drive is less effective for this reason as well.
In order to be able to operate a linear compressor in a stable way with a high degree of efficiency, it is therefore necessary to monitor the amplitude of the armature and to control the alternating current applied to the winding in such a way that the amplitude always remains just under a limit value the exceeding of which causes the piston to strike a boundary.
Tolerances during the production of linear compressors can mean that the path which the armature is able to cover from its equilibrium position until the piston strikes a boundary can vary from one linear compressor to another. If, taking into account the production tolerances, the armature stroke is defined uniformly for all linear compressors so that the piston is not able to strike the boundary, the dead volumes differ greatly from one compressor to another and hence so does the efficiency.
A further problem is that the equilibrium position adopted by the armature when the compressor is switched off can differ depending upon the pressure acting on the piston and prevailing in the compression chamber. When using the linear compressor to compress refrigerants in a refrigerator, different pressures can easily occur depending on the average temperature or the ratio of gaseous to liquid refrigerant in the device's refrigerant circuit. When a refrigerator is put into operation for the first time or put into operation after a lengthy outage period and the refrigerant circuit has to be cooled down from room temperature, at first the pressure in the refrigerant circuit is higher than it is with an operational device in which the refrigerating compartment, and consequently also at least a part of the refrigerant, is much colder than room temperature. An oscillation amplitude which produces a small usable dead volume with an operational device can be insufficient in the case of new commissioning, since here the rest position about which the armature is oscillated is displaced. If this results in a large dead volume, in extreme cases, the efficiency of the compressor can be so greatly impaired that it is not possible to cool down the device in the correct manner.